Revamp Schema declaration

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt

@@ -158,15 +158,16 @@ internal fun FunctionDeclaration.toInternal() =
     name,
     description,
     Schema(
-      properties = getParameters().associate { it.name to it.toInternal() },
-      required = getParameters().map { it.name },
+      properties = parameters.mapValues { it.value.toInternal() },
+      required = parameters.keys.minus(optionalParameters.toSet()).toList(),
       type = "OBJECT",
+      nullable = false,
     ),
   )
 
-internal fun <T> com.google.firebase.vertexai.type.Schema<T>.toInternal(): Schema =
+internal fun com.google.firebase.vertexai.type.Schema.toInternal(): Schema =
   Schema(
-    type.name,
+    type,
     description,
     format,
     nullable,

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/FunctionDeclaration.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.firebase.vertexai.type
+
+/**
+ * A declared function that a model can be given access to in order to gain info or complete tasks.
+ *
+ * ```
+ * val getExchangeRate = FunctionDeclaration(
+ *    name = "getExchangeRate",
+ *    description = "Get the exchange rate for currencies between countries.",
+ *    parameters = mapOf(
+ *      "currencyFrom" to Schema.str("The currency to convert from."),
+ *      "currencyTo" ot Schema.str("The currency to convert to.")
+ *    )
+ * )
+ * ```
+ *
+ * @param name The name of the function call, this should be clear and descriptive for the model.
+ * @param description A description of what the function does and its output.
+ * @param parameters A list of parameters that the function accepts.
+ * @param optionalParameters A list of parameters that can be omitted.
+ * @see Schema
+ */
+class FunctionDeclaration(
+  val name: String,
+  val description: String,
+  val parameters: Map<String, Schema>,
+  val optionalParameters: List<String> = emptyList(),
+)